This invention relates to a method of controlling to reduce or eliminate microorganisms by exposing them to ultraviolet (UV) radiation and, more particularly, to a method of surface microbial disinfection of foods and other objects subject to microbial contamination by using ultraviolet (UV) radiation with short high-intensity pulses.
Because of the current trends in consumer demands and regulatory actions, there is an urgent need to develop non-additive alternative processes capable of replacing or minimizing the food industry's current reliance on many chemical preservatives used as antimicrobial agents. A majority of these agents exert their controlling action on the microbial flora present on the surface of foods. Under exposure to non-ionizing radiation energy, microbial control has been shown to be caused by structural alterations in nucleic acids such as desoxyribonucleic acid (DNA) and ribonucleic acid (RNA), as well as in other chemical structures such as proteins and enzymes having similar resonance frequencies and high absorptivity for UV radiation. These alterations prevent DNA replication and transcription and hence the growth of the microbial population. No information, however, has been available yet about the effectiveness of this process on contaminated food surfaces, nor has it been known whether unwanted changes of the sensory properties of foods may also occur.
A majority of the research, reported thus far, on effects on DNA induced by non-ionizing radiation has been based on the use of Hg lamps emitting 260 nm (4.77 eV) ultraviolet radiation. The inactivation of DNA under this condition, therefore, is well known. Low-intensity (0.10-10W/m.sup.2) continuous-wave polychromatic (broad band) ultraviolet irradiation (4.88 eV) of DNA and RNA, nucleic acid components including purine and pyrimidine bases, as well as the bases themselves, leads to one-quantum excitation of low-lying electronic energy levels of the targeted molecule. This one-step process is due to the absorption of one photon. Under these conditions, radiation damage to DNA results from dimerization and/or hydration of adjacent pyrimidine bases. Furthermore, the dimer may not be repaired by photoreactivation or dark reactivation. It blocks the enzymatic action of DNA-polymerase and hence inhibits DNA replication. Although effective, this low-intensity process is extremely slow and limited for large-scale microbial controls from logistical and economical points of view.
Lasers generating monochromatic ultraviolet radiation with short pulses in the range of picoseconds to nanoseconds, and hence with considerably more radiation power than continuous-wave polychromatic ultraviolet lamps, have also been studied as sources for DNA effects. With the development of high-intensity ultraviolet laser radiation sources, it was shown that an effective destruction of nucleic acids resulted in vitro when aqueous solutions containing nucleic acid components were exposed thereto. It has also been found recently that high-power ultraviolet laser radiation causes two-photon photolysis of water and that the products of water photolysis, as well as products of water radiolysis from ionizing radiation sources, could also react effectively with nucleic acid components. The efficiency of indirect photolysis, however, was also found to be 2-4 times lower than that of radiolysis, suggesting the possibility of exploring lower wavelength ultraviolet laser radiation to approach water radiolysis yields. The latter approach was demonstrated when induction of single-strand scissions in Simian Virus 40 DNA were detected when exposing it to pulsed (20 nsec) ultraviolet laser radiation of 193 nm (6.42 eV). These results were comparable to the single-strand and double-strand scissions typical of ionizing radiation.
Extensive research has also been conducted with sources of ionizing radiation. Most of these investigations, however, were based on the use of 1.2-MeV Co-60 and 0.662-MeV Cs-137 photons from radioisotopic sources, bremsstrahlung radiation or electron beams from an accelerator. Currently, world-wide acceptance of the technology of food irradiation is growing. Since viruses, mycoplasmas, bacteria and fungi can be destroyed by ultraviolet radiation, whether they are suspended in air or in liquids or deposited on surfaces, ultraviolet radiation has been used in a variety of applications such as (1) destruction of air-borne microorganisms for improving air-hygiene, (2) inactivation of microorganisms located on surfaces or suspended in liquids, and (3) protection or disinfection of many products of unstable composition that cannot tolerate other conventional treatments such as by heat, gas or chemicals.